The present invention generally relates to an atrial tracking cardiac stimulator. The present invention more particularly relates to an implantable atrial tracking cardiac stimulator or pacemaker which senses atrial activity and paces the ventricles responsive to selected sensed atrial activations in order to regulate ventricular rates in the presence of high or increasing atrial rates. The ventricular rate regulation is seamless without requiring pacing modality modification.
Implantable pacemakers are well known in the art. Early pacemakers were single chamber pacemakers which only paced the ventricles in a trigger mode. They did not sense any cardiac activity and paced the ventricles at a pre-determined, fixed rate.
Later single chamber pacemakers both sensed ventricular activity and paced the ventricles. The ventricular sensing allowed the pacemaker to inhibit pacing when a spontaneous ventricular activation (R wave) was sensed within an escape interval corresponding to the fixed pacing rate. Such pacing is referred to as demand pacing since the heart is paced only when necessary. This pacing modality is referred to in the art as VVI pacing.
As the pacemaker art advanced, dual chamber pacemakers were made available. The first dual chamber pacemakers sensed in both the atria and ventricles and paced the ventricles. These dual chamber pacemakers were primarily for heart block patients who suffered from lack of conduction between the atria and ventricles. Their purpose was to simulate normal atrial-ventricular synchrony in heart block patients by coupling ventricular response to atrial activity. When an atrial activation (P wave) was sensed, it started the timing of an AV delay. At the end of the AV delay, the ventricles were paced. The most significant benefit of the foregoing was that when the atrial rate increased due to exercise or some other cause of increased metabolic demand, the ventricular rate would similarly increase so that the hemodynamic output of the heart would satisfy the metabolic demand. Such pacemakers could also function in a demand mode supported by ventricular sensing.
Atrial pacing was later added to the capabilities of dual chamber pacemakers. These pacemakers are referred to in the art as DDD pacemakers. They not only assist heart block patients by coupling the atria and ventricles, but further promote atrial function in sick-sinus syndrome patients whose atria generally do not function properly on their own.
While current dual chamber pacemakers have done much to treat cardiac patients, and especially those with sick-sinus syndrome and heart block, such devices are not without their problems. For example, when a heart block patient experiences a rapid atrial rate, a dual chamber pacemaker will pace the ventricles at the high atrial rate due to the coupling between the atria and the ventricles. If the high atrial rate is not due to metabolic demand, but instead due to an atrial tachyarrhythmia, such as atrial flutter or atrial fibrillation, the ventricular rate will be higher than required to meet metabolic demand. In a sense, the pacemaker would drive the heart too fast for these conditions.
To overcome the above problems associated with DDD pacing and the presence of atrial tachyarrhythmias, mode switching was added to these devices. The mode switching would cause the pacemaker to switch from a DDD mode to a VVI mode set to pace at an essentially normal rate. An abrupt change in ventricular rate can occur at the point of the mode switch which many patients find objectionable. One such DDD pacemaker is described in U.S. Pat. No. 4,624,260 which mode switches from the DDD modality to the VVI modality in response to high atrial rates.
Another problem with DDD pacemakers is that by their very nature, together with the human physiology of the heart, the pacemaker can cause a fast rate on its own. This condition is known as a pacemaker mediated tachycardia (PMT) and occurs when the pacemaker senses a P wave which resulted from a paced or spontaneous ventricular activation retrogradedly conducted to the atrium. As a result, the retrogradedly conducted R wave is sensed as a P wave and initiates a device AV pacing delay. At the end of the AV delay, the ventricle is paced to set up another retrogradedly conducted R wave. If this is allowed to persist, a very fast heart rate can result. To preclude and/or terminate a PMT, dual chamber pacemakers incorporate additional PMT prevention and/or termination functionality.
The present invention represents a further improvement in the evolution of the dual chamber pacemaker. It tracks the atrial rate. However, through a unique rejectory processor, it paces the ventricles after an AV interval for only selected ones of the detected P waves for regulating the ventricular rate in the presence of a high atrial rate. It accomplishes this end seamlessly without requiring mode switching. Further, pacemaker mediated tachyarrhythmias are automatically precluded thus rendering additional PMT prevention and/or termination functionality unnecessary.